You're going to want this felpro gasket, as well as these o-rings. Find them. Buy them.
The o-rings are standard -015 o-rings. Pay attention to materials and durometer. You want a 70 or harder durometer and a material that will withstand oil and 300+deg F.
Step 1: Jack It Up.
Jack up your vehicle with the driver's side lower than the passenger's side (unless you want to change your oil, then it really doesn't matter). Use jack stands; unless you have a death wish.
Step 2: Remove Drain Plug.
Put a 1/4" ratchet into the coolant drain plug hole at the high point/crossover pipe and remove the plug.
Step 3: Drain Some Coolant.
Put a bucket under the radiator, attach a hose if you want to be pro, and open the petcock valve. Coolant will drain for about a half hour. Go have lunch.
Step 4: Pull Coolant Hose.
Use your vice grips or channel lock pliers, and relocate the spring hose clamp on the coolant hose in question. Put a bucket or pan under this hose area, because coolant will be lost. Yes, you thought you drained it all. Too bad, you didn't. Pry, Pull, Prod, twist, cuss, etc until hose comes loose. Put it out of the way. Make a mess. Be happy.
Step 5: Remove!
If you're going to reuse coolant, it's time to relocate it because you're about to leak oil and make a coolant/oil mix. Using a new catch pan, unscrew the oil filter and deal with the oil mess. Unplug the oil pressure sensor. Push hard on the little rectangular tab in the middle and pull. Next, Using a 13mm socket and as many 3/8" drive extensions as you can find, remove the 4 bolts in question. They are tight. Mine were over 70ft-lbs tight. They are only supposed to be 22ft-lbs, which is probably why it leaked. Pry the casting loose, and you'll find it dribbles yet more coolant. Since you jacked the car up at an angle, none enters the engine. Your oil is fine. If you don't believe it, drain the oil and check it yourself. I've done this 4 times using this method and have found not a single drop of water in the oil. Remove the adapter casting and clean it, as well as the mating surfaces where the new gasket goes.
Step 6: Install.
With both surfaces clean and smooth, there is NO NEED FOR SEALANT. If you have pitted surfaces, you have no choice but to use sealant. I recommend using a product called "the right stuff" in an aerosol can. Use sparingly, you don't need a lot and you don't want it blocking the water passages. In my case, I had clean shiny surfaces and used NO SEALANT. Start by zip-tying the gasket to the casting using two bolt holes, put a bolt another hole, and get it started. Once you succeed, insert the 2nd bolt then clip both of the zip ties and pull them free before adding the last 2 bolts and tightening to spec. Spec is 22ft-lbs. Not 42. Not 82. 22 is the number. Over-do it and you'll crush the gasket and it'll leak. Check the torque on every bolt twice. Spin the oil filter back on. Reconnect the oil pressure sensor. Stick the radiator hose back on the housing.
Step 7: Fill
Wedge a funnel in the filler. Take the cap off the radiator overflow tank. Pour in coolant. It's going to take about 3 gallons. When you get to about 2 gallons, watch the overflow level. When it rises to the right mark, close the overflow tank cap to prevent it from rising further. Add coolant until the funnel is full of coolant. Start the car. Let it warm up to operating temp. Add coolant as needed. It will burp/bubble/spout/make a mess. The belts will squeak as they get lubricated with coolant. Clean the engine bay while you're waiting. It'll take a half hour or more to warm up and burp the system.
Step 8: Cap Off System
Install a new o-ring on the crossover cap, slather it with antisieze, and install it. Take it for a test drive. Watch the temp gauge. Park it. Let it cool off. Pull the crossover cap again and top off the coolant. Check it again like this before you drive it for the next few days until you find that it's completely full at the crossover every time you check it. Congrats, you're done.